The present invention relates to an optical attenuator for attenuating a light signal of optical communications, optical measurement, CATV, and other systems and a method of making the same, an optical fiber cable incorporating the optical attenuator and a method of making the same.
FIG. 15 is a cross-sectional view showing a conventional optical attenuator.
The conventional optical attenuator 50 comprises an optical fiber 1a having a core 2a covered with a clad 3a, and an optical fiber 1b having a core 2b covered with a clad 3b butted and joined at their ends to each other so that the center of one core is dislocated from the center of the other by a distance L' which is determined depending on the diameter of the cores 2a, 2b and a desired level of attenuation.
The joining is carried out by an arc discharge method. The arc discharge may be performed for e.g. three seconds.
FIG. 16 is a cross-sectional view showing a conventional optical attenuator disclosed in Japanese Utility Model Publication No.7-46881 (1995).
The optical attenuator 60 is made by butting and joining one end of a capillary 62a covering an optical fiber 61a and one end of a capillary 62b covering an optical fiber 61b which is stepped on an end surface thereof to each other, and then inserting an attenuation film 63 into a gap formed between the two capillaries 62a and 62b.
In the conventional optical attenuator 50, an area where the core 2a (or 2b) is directly communicating with the clad 3b (or 3a) at the joint portion between the two optical fibers 1a and 1b is formed. A transmitting light is reflected back by the area to the input side. If the reflectance is high, the transmitting light reflected back to an optical oscillator, such as a laser device, will make oscillation unstable and produce noise.
The level of attenuation is determined by the distance L' between the optical axes of the two cores 2a and 2b. However, the diameter of the cores 2a and 2b is too small to control variation in the distance L', causing the level of attenuation to be set with less accuracy.
Also, as the optical centers of the two optical fibers are dislocated from each other, their joint portion is stepped and will be fitted into a connector with difficulty.
In the conventional optical attenuator 60, there is a problem that a production process becomes complex, since the end of the capillary 61b is to be stepped by a milling process. Also, there is a problem that an accuracy of attenuation is low because of error in the milling process.